This study has two parts. The first part will document the melatonin profile by saliva samples in a 48 hour period in approximately 60 patients (aged 3 to 21 years) with fragile X syndrome (FXS) and 60 controls with mental retardation including those with Angelman syndrome, Velocardiofacial syndrome, and Smith Magenis syndrome in addition to normal controls including normal siblings. These patients will be enrolled from participating centers including Melbourne, Lille, France, NIH and Denver. The second part of the study is a three year project to evaluate the efficacy of melatonin compared to placebo in the treatment of sleep disturbances in patients with FXS or other causes of mental retardation. This involves a two week baseline run in with four weeks of treatment with either melatonin or placebo which is crossed over for a second four week period. Sleep diaries will be used to document awakenings and saliva melatonin levels will be measured at two week intervals to document compliance and peak levels after an oral dose. Patients will be enrolled in Denver and other centers noted above.